Comet Snow Storm

Deep Impact finished its prime mission in 2005, when it successfully delivered an impactor to comet Tempel 1. The spacecraft, renamed EPOXI, is now heading for comet Hartley 2. Credit: NASA

NASA spacecraft sees cosmic snow storm during comet encounter

The EPOXI mission’s recent encounter with Comet Hartley 2 provided the first images clear enough for scientists to link jets of dust and gas with specific surface features. NASA and other scientists have begun to analyze the images.

The EPOXI spacecraft revealed a cometary snow storm created by carbon dioxide jets spewing out tons of golf-ball to basketball-sized fluffy ice particles from the peanut-shaped comet’s rocky ends. At the same time, a different process was causing water vapor to escape from the comet’s smooth mid-section. This information sheds new light on the nature of comets and even planets. Studying the materials present in comets can help astrobiologists understand their potential role in the origin of life during periods on the early Earth when cometary impacts were more frequent.

Scientists compared the new data to data from a comet the spacecraft previously visited that was somewhat different from Hartley 2. In 2005, the spacecraft successfully released an impactor into the path of Comet Tempel 1, while observing it during a flyby.

This image from the High-Resolution Instrument on NASA’s EPOXI mission spacecraft shows part of the nucleus of comet Hartley 2. Image Credit: NASA/JPL-Caltech/UMD

“This is the first time we’ve ever seen individual chunks of ice in the cloud around a comet or jets definitively powered by carbon dioxide gas,” said Michael A’Hearn, principal investigator for the spacecraft at the University of Maryland. “We looked for, but didn’t see, such ice particles around Comet Tempel 1.”

The new findings show Hartley 2 acts differently than Tempel 1 or the three other comets with nuclei imaged by spacecraft. Carbon dioxide appears to be a key to understanding Hartley 2 and explains why the smooth and rough areas scientists saw respond differently to solar heating, and have different mechanisms by which water escapes from the comet’s interior.

“When we first saw all the specks surrounding the nucleus, our mouths dropped,” said Pete Schultz, EPOXI mission co-investigator at Brown University. “Stereo images reveal there are snowballs in front and behind the nucleus, making it look like a scene in one of those crystal snow globes.”

Data show the smooth area of Comet Hartley 2 looks and behaves like most of the surface of Comet Tempel 1, with water evaporating below the surface and percolating out through the dust. However, the rough areas of Hartley 2, with carbon dioxide jets spraying out ice particles, are very different.

This image shows the nuclei of comets Tempel 1 and Hartley 2, as imaged by NASA’s Deep Impact spacecraft, which continued as an extended mission known as EPOXI. Image Credit: NASA/JPL-Caltech/UMD

“The carbon dioxide jets blast out water ice from specific locations in the rough areas resulting in a cloud of ice and snow,” said Jessica Sunshine, EPOXI deputy principal investigator at the University of Maryland. “Underneath the smooth middle area, water ice turns into water vapor that flows through the porous material, with the result that close to the comet in this area we see a lot of water vapor.”

Engineers at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., have been looking for signs ice particles peppered the spacecraft. So far they found nine times when particles, estimated to weigh slightly less than the mass of a snowflake, might have hit the spacecraft but did not damage it.

“The EPOXI mission spacecraft sailed through the Hartley 2’s ice flurries in fine working order and continues to take images as planned of this amazing comet,” said Tim Larson, EPOXI project manager at JPL.

Scientists will need more detailed analysis to determine how long this snow storm has been active, and whether the differences in activity between the middle and ends of the comet are the result of how it formed some 4.5 billion years ago or are because of more recent evolutionary effects.